I went from a 450D to a 50D, although I think the 50D is discontinued & replaced by the 60D. It was an easy learning curve to the 50D. The 7D is a more advanced camera, but is very similiar to the 50D also.
Me too, I went from the 400D to a second hand 40D, second hand is always a good option depending on your budget. The 70D is the latest in that range and is quite a good camera, if going the second hand route a good 7D would be your best bet.